The sound engineering practice of taking soil (electrolyte) resistivity measurements prior to designing a cathodic protection (CP) system is often overlooked, frequently underused or is a neglected practice. Often, designers are asked to guess or “use [this] information that was collected a few miles away” or “the soil resistivity in this area is all the same, just use [insert value here]”. Believe it or not, cathodic protection systems are installed all the time without any knowledge about what they are being installed into.
Unfortunately, the full appreciation of the matter is missed until a cathodic protection system is installed (either galvanic or impressed current) and the anticipated current outputs cannot be obtained. If the output is not there, it is likely the verifications will take place; connections, installation, wires, etc. until the “oh no!” moment. At this point it is realized that skipping soil resistivity testing can have a large impact on the total time and cost of the project, considering it is likely a new ground bed may be necessary; either to replace or supplement the underperforming groundbed.
What does a soil resistivity test tell you?
Soil resistivity testing determines the conductivity of the soil and semi-directly relates to the amount of corrosion activity you may have on a buried structure. It can also be used to dictate the type of cathodic protection system you would want to use for your application.
What do you do with the information?
Soil resistivity has a direct and indirect impact on almost everything within a cathodic protection system. This information is used to anticipate CP system lifespan, structure CP current requirements, size of the anode bed, anode bed spacing and power supply size.
And that’s the short list.
Where is cathodic protection used?
There are many diverse applications for cathodic protection. Many of them are happening all around, usually operating silently in the background. A few examples of these applications are:
- Steel Municipal Water Pipelines
- Steel and Stainless-Steel Liquid Fuel, Oil and Natural Gas Pipelines
- Steel Acetic Acid, Hydrogen and CO2 Pipelines
- Steel Storage Tanks
- Steel Pier Piles
- Hulls of Ships and Boats
- Offshore Oil Platforms
- Onshore Oil Well Casings
- Windfarm Foundations
- Reinforced Concrete in Bridges, Buildings and Other Structures
How do we test the soil resistivity?
On the ground, in-situ resistivity testing is performed using the Wenner 4-Pin Method in accordance with ASTM G57. This test method uses four metallic pins driven into the soil in a straight line at equidistant spacing. A ground resistance tester is used to discharge current usually alternating into the soil from the two outer pins. As the electrical current flows between the two outer pins a voltage gradient is formed in the soil proportional to the average resistance of the soil. The voltage drop between the two inner pins is measured, and the average resistance of the soil down to a depth equal to the pin spacing is reported by the ground resistance tester. Performing multiple measurements at different surface spacings provides a relatively accurate indication of the soil strata.
What does this mean to you?
Taking soil resistivities to optimize the information for cathodic protection design will help mitigate the risk of rework. Making sure the necessary data is gathered will improve the chances of executing a successful project. At TRC, we have a clear process in place to ensure that all our teams follow the best practices we have just outlined, which in turn ensures that we don’t make mistakes that cost our clients unnecessary increases in cost and time.